1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus having the same, and more particularly to structure for separating and feeding sheets having high adhesion properties between those sheets.
2. Related Background Art
A conventional image forming apparatus such as a copying machine and a printer has had a sheet feeding apparatus which after feeding out sheets stacked on sheet stacking means one sheet at a time from one at the uppermost position in order by a pickup roller which is sheet feeding means, separates one sheet at a time by a separating portion for feeding to an image forming portion.
In this case, when continuous sheets feeding is performed in such a sheet feeding apparatus, cut sheets are used, and such cut sheets have been normally limited to woodfree paper or standard sheets designated by the copying machine manufacturer. Also, in order to reliably separate such sheets one sheet at a time for feeding, various separating methods have been conventionally adopted, and as such a separating system, there is, for example, a separating pad system in which a frictional member is caused to abut against a feed roller at predetermined pressure for preventing double feeding.
Also, as another separating system, there is a retard separating system in which a feed roller for rotating in a sheet conveying direction and a separating roller which is driven in a direction opposite to the sheet conveying direction at predetermined torque and abuts against the feed roller at predetermined pressure constitute a separating portion, and by this separating portion, only the sheet at the uppermost level of a sheet stack sent out by the pickup roller is caused to pass through, and another sheet which has been accompanied by the uppermost sheet and fed is returned on the sheet stacking means side to thereby prevent double feeding.
In this case, in order to reliably separate and feed sheets by these separating systems, in the case of, for example, the retard separating system, it has become possible to reliably separate sheets one sheet at a time by optimizing the return torque of the separating roller and the applied pressure by taking into consideration the frictional force of the sheet to be fed.
In recent years, along with the diversification of sheets (recording media), there has been growing a request for forming an image also on a sheet such as a coated sheet obtained by performing coating processing on the surface of a sheet in order to give brightness or a gloss from the request from the market for colorization in addition to ultra-thick paper, an OHP sheet, art film and the like.
When the ultra-thick paper is going to be fed, however, the dead load of the ultra-thick paper becomes conveying resistance and the paper cannot be picked up, but paper jam occurs. Also, sheets made of resin material easy to be charged with electricity as in the case of the OHP sheet or the art film are incapable of picking up or cause double feeding because during a feeding operation under a low humidity environment, sheets rub against each other, whereby their sheet surfaces are gradually charged with electricity and the sheets come to stick to each other by Coulomb forces.
Also, in coated sheets obtained by coating the surface of a sheet with coating material consisting of paint or the like, since the coated sheets have a property that the sheets attract each other particularly when stacking under a high humidity environment, there arises a problem that pickup cannot be performed and double feeding occurs frequently.
This is because although in the case of such a specialty sheet as described above, a frictional force itself between the sheets themselves is equal to or less than that of standard sheet and the like, in the case of resin material sheets, by means of an attraction force due to frictional charging under the low humidity environment, and in the case of the coated sheet, by means of an attraction force under the high humidity environment, the attraction is performed by a far higher force than the frictional force between the sheets themselves, and therefore, separation can be hardly made by the conventional separating system.
In other words, in the conventional separating system, since only the frictional force between the sheets themselves has been taken into consideration, the sheets cannot be reliably separated when such an attraction force other than the frictional force is exerted.
Thus, in order to nullify such a very high attraction force between the sheets themselves, there is a separation feeding system in which as disclosed in Japanese Patent Application Laid-Open No. H11-005643, air is blown from the side of the sheet stack to ravel the sheets in advance; in a state in which any attraction between the sheets has been nullified, the sheets are picked up one sheet at a time from a sheet at a higher position; and the sheets are separated one sheet at a time at a separating portion provided at the downstream portion. Apparatuses in which this separation feeding system has been adopted have been used in the printing business world or in some copying machines.
In the separation feeding system having means (hereinafter, referred to as auxiliary air raveling means) for blowing air from the side of such a sheet stack, since even sheets (recording media) having such a high attraction force as described above nullify the attraction by raveling the sheets prior to feeding, the separation performance has been improved as compared with the system utilizing only the frictional force described above.
FIG. 15 is a view showing structure of a sheet feeding apparatus having such auxiliary air raveling means, and this sheet feeding apparatus 155 has: a sheets feeding tray 59 for stacking sheets S; sheet feeding means (not shown) for sending out a sheet S from the sheets feeding tray 59; air blowing means 71 for blowing air on the side of the sheets S stacked; and flow path moving means 157 for moving the air blowing means 71 in the vertical direction along the side of the sheet S in a vertical direction.
The flow path moving means 157 has: a guide rail (not shown) for supporting the air blowing means 71 in a perpendicular direction in such a manner as to be freely movable; an electrical motor 121; and a cam plate 123 fixed to the output shaft of this electrical motor 121, for moving the air blowing means 71 by slidably contacting the underside of the air blowing means 71.
In such a flow path moving means 157, when the electrical motor 121 rotates, the cam plate 123 causes the air blowing means 71 to move in the vertical direction, and accordingly, an air course is to move in the vertical direction. In this case, since an opening (air duct port) of the air blowing means 71 has an opening area which is constant always, when the air blowing means 71 lowers, the side of the sheet S is to face the opening, whereby the area of the opening is reduced, and air to be blown off from the opening is to be narrowed down. As a result, it becomes possible to nullify the attraction between all the sheets by causing the sheet S at a higher position to float in the air.
In this respect, as the separation feeding system in which air has been arranged to be flown from the side of the sheet stack as described above, there is a sheet feeding apparatus in which as disclosed in Japanese Patent Laid-Open No. 2001-48366, the blown air is heated by a heater to thereby dehumidify the sheets and the attraction force of the sheets (coated sheets) particularly under a high humidity environment has been mitigated.
In the sheet feeding apparatus in which the separation feeding system for blowing air from the side of the sheet stack has been adopted, however, when air is blown, only the neighborhood of the air blowoff port of the sheets stacked is partially dried particularly under a low humidity environment.
Thus, when the sheets are partially dried as described above, unevenness develops in the surface resistance value within the sheet surface, and as a result, when the sheet is fed to an image forming portion of the image forming apparatus, this dried portion causes a transfer failure, resulting in an image defect. Particularly, in the case of an electrophotography system in which the image forming portion transfers a toner image on the sheet through the use of static electricity, since the transfer performance greatly depends upon the surface resistance value of the sheet surface, when unevenness develops in the surface resistance value, transfer unevenness occurs, image deterioration due to it is conspicuous, and becomes very unsightly.